I have a preban Colt small pin rifle. A friend is giving me a carbine barrel assembly that is preban with the birdcage fh on it. It is the barrel, handguards, etc. but doesn't include the entire upper so I can't just align the pin and go.

I presume it is everything from the handguards forward although I haven't examined it closely and don't know that much about it anyway. Presuming that is correct and it's the barrel etc. what else do I need and how much am I looking at spending to make it ready to swap with my rifle length upper? I don't want to scavenge any parts off the rifle upper just in case I want to switch back at some point. Thanks for any input on this.

You can refer to this diagram:
[img]http://www.ar15.com/content/parts/upper.gif[/img]

Which shows all the parts in an upper receiver.

Assuming, (a dangerous thang to do) that your barrel is complete, it will have gas-tube, Delta Ring (#8), the snap-ring (#6) and the spring (#7), so all you will need to complete it will be a:

complete upper receiver
a bolt
a bolt carrier
a charging handle

You'll need an Armorer's Vise like this, to hold the upper receiver:
[img]http://www.bushmaster.com/shopping/gunsmith/Images/om-003-withupper.jpg[/img]

and an AR-15/M-16 Armorer's tool like this:
[img]http://www.bushmaster.com/shopping/gunsmith/Images/223-wrench.jpg[/img]

Perhaps a little moly-grease or anti-sieze compund to put on the threads and a torque wrench (calibrated in "inch" pounds) which will fit into the armorer's wrench.

Hope this helps, BTW there is good info both on the homepage here, (where diagram comes from):
[url]http://www.ar15.com/content/parts/upper.html[/url], and LOTS of excellent info here on "how-to" type subjects: [url]http://groups.msn.com/TheMarylandAR15ShootersSite/[/url]

Mike